Liquid crystal displays have been widely used for monitor for personal computer and cellular phone, television, etc. because they are advantageous in that they can operate at low voltage with low power consumption and are available in small size and thickness. These liquid crystal displays have been proposed in various modes depending on the alignment of liquid crystal molecules in the liquid crystal cell. To date, TN mode, in which liquid crystal molecules are aligned in twisted at about 90 degrees from the lower substrate to the upper substrate of the liquid crystal cell, has been a mainstream.
A liquid crystal display normally comprises a liquid crystal cell, an optical compensation sheet and a polarizing film (or a polarizer). The optical compensation sheet is used to eliminate undesirable coloring of image or expand the viewing angle. As such an optical compensation sheet there is used a stretched birefringent film or a transparent film coated with a liquid crystal. For example, Japanese Patent No. 2,587,398 discloses a technique for the expansion of the viewing angle involving the application to a TN mode liquid crystal cell of an optical compensation sheet obtained by coating a discotic liquid crystal over a triacetyl cellulose film, and then orienting (or aligning) and fixing the coat layer. However, liquid crystal displays for TV use which are supposed to give a wide screen image that can be viewed at various angles have severe requirements for dependence on viewing angle. These requirements cannot be met even by the aforementioned approach. To this end, liquid crystal displays of modes different from TN mode, including IPS (In-Plane Switching) mode, OCB (Optically Compensatory Bend) mode, VA (Vertically Aligned) mode, have been under study. In particular, VA mode has been noted as liquid crystal display for TV use because it gives a high contrast image and can be produced in a relatively high yield.
A cellulose acylate film is characterized by a higher optical isotropy (lower retardation value) than other polymer films. Accordingly, it is normally practiced to use a cellulose acylate film in uses requiring optical isotropy such as polarizing plate.
On the contrary, the optical compensation sheet (retardation film) for liquid crystal display is required to have optical anisotropy (high retardation value). In particular, the optical compensation sheet for VA mode is required to have a front retardation (Re) of from 30 to 200 nm and a thickness direction retardation (Rth) of from 70 to 400 nm. Accordingly, it has been usually practiced to use, as an optical compensation sheet, a synthetic polymer film having a high retardation value such as polycarbonate film and polysulfone film. The front retardation value and the thickness direction retardation value are optical properties calculated by formulae (V) and (VI), respectively.Re=(nx−ny)×d  (V)Rth={(nx+ny)/2−nz}×d  (VI)wherein nx represents the refractive index in x direction of the film plane; ny represents the refractive index in y direction of the film plane; nz represents the refractive index of the film in the direction perpendicular to the film plane; and d represents the thickness (μm) of the film.
As mentioned above, it is a general principle in the art of optical material that a synthetic polymer film is used in the case where a polymer film having a high optical anisotropy (high retardation value) is required while a cellulose acylate film is used in the case where a polymer film having an optical isotropy (low retardation value) is required.
EP 0911656 A2 overthrows this conventional general principle and proposes a cellulose acylate film having a high retardation value that can be used also for purposes requiring optical anisotropy. In accordance with this proposal, an aromatic compound having at least two aromatic rings, particularly a compound having 1,3,5-triazine ring, is added to cellulose triacetate to be stretched in order to realize a cellulose triacetate film having a high retardation value. It is generally known that a cellulose triacetate is a polymer material that can be difficultly stretched and provided with a high birefringence. However, EP 0911656 A2 proposes that when additives are oriented at the same time with stretching, making it possible to raise birefringence and realize a high retardation value. This film is advantageous in that it can act also as a protective film for polarizing plate and thus can provide an inexpensive thin liquid crystal display.
JP-A-2002-71957 discloses an optical film having as a substituent a C2-C4 acyl group that satisfies formulae 2.0≦A+B≦3.0 and A<2.4 at the same time supposing that the degree of substitution by acetyl group is A and the degree of substitution by propionyl group or butyryl group is B, wherein the refractive index Nx of the film in the direction of slow axis and the refractive index Ny of the film in the direction of fast axis at a wavelength of 590 nm satisfy formula 0.0005≦Nx−Nz≦0.0050.
JP-A-2003-270442 discloses a polarizing plate for use in VA mode liquid crystal display, wherein the polarizing plate has a polarizing film and an optically biaxial mixed aliphatic acid cellulose ester film which is disposed interposed between the liquid crystal cell and the polarizing film.
The methods disclosed in the aforementioned references are advantageous in that inexpensive thin liquid crystal displays can be obtained. In recent years, however, there has been a rapidly growing tendency for more liquid crystal displays to be larger in size. In particular, since the mixed aliphatic acid cellulose ester film disclosed in JP-A-2002-71957 and JP-A-2003-270442 has a low elastic modulus, a large thermal expansion coefficient and a large hygroscopic expansion coefficient, polarizing plates comprising the mixed aliphatic acid cellulose ester film as a protective film capable of compensating the viewing angle of liquid crystal cell can curl much, causing a problem that malsticking can easily occur at a step of sticking the polarizing plate to the liquid crystal cell.
It has been thus desired to develop a polarizing plate having an optical compensation capacity which causes no such defectives at a step of sticking a polarizing plate to a liquid crystal cell.